DC backup power supply system

ABSTRACT

A DC backup power supply system having a battery; a charge-discharge circuit for charging and discharging a power between the battery and a DC line; and a control circuit for controlling the charge-discharge circuit, wherein the battery has a number of battery cells and cylindrical portions of the battery cells are laid on an approximately horizontal plane.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application relates to subject matters described inthe co-pending U.S. patent application Ser. No. 10/083,638 filed on Feb.27, 2002 and a U.S. patent application serial No. which is filed on thesame date of the present application and based on Japanese patentapplication No. 2002-113117 filed on Apr. 16, 2002 in Japan.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a DC backup power supply system,and more particularly to a DC backup power supply system having acharge-discharge circuit for charging and discharging power between abattery and a DC line.

[0003] An uninterruptible power system (UPS) is externally installed ona so-called information processing apparatus such as a server, a routerand a storage to deal with an unexpected power failure of a commercialAC power supply system and avoid damages such as data loss to be causedby the power failure. A rack-mount type UPS is commercially availablewhich can be mounted on a rack having a width of about 480 mm called a19-inch rack for information processing apparatuses. This rack has sucha dimension as described, for example, in a “Smart-UPS” catalog of APCJapan, Ltd. This UPS is an AC backup power supply system which suppliesan AC output power from a rechargeable battery to a load via an inverterand a transformer to retain the load operation.

[0004] Japanese Patent Laid-open Publication JP-A-2000-197347 disclosesa DC backup power supply system to be used for a system in which a DCpower is supplied from an AC power supply system to a load via an AC/DCconverter and a DC/DC converter. In this Publication, it is proposed toconnect the DC backup power supply system to an intermediate DC linebetween both the converters in order to increase the conversionefficiency and reduce the volume and cost.

[0005] Although a seal type lead battery generally used as arechargeable battery of UPS is relatively inexpensive, it has a largevolume and is difficult to be mounted on an information processingapparatus or the like. In addition, in order to retain the reliabilityof an information processing apparatus or the like, double or tripleredundancy of a backup power supply system is required so that the sizeof UPS becomes larger and the mount issue becomes more serious.

[0006] Since a seal type lead battery contains lead, it is associatedwith the problem of adverse affects upon the environments if it isdumped as lead waste.

SUMMARY OF THE INVENTION

[0007] It is an object of the present invention to provide a compact DCbackup power supply system. Specifically, a DC backup power supplysystem having a capacity of 1 to 1.4 kVA is made as compact as 45 mmheight or lower to be able to be mounted in a 19-inch rack having a oneunit (1U) size.

[0008] According to a first aspect of the present invention, a DC backuppower supply system is provided which comprises: a battery; acharge-discharge circuit for charging and discharging a power betweenthe battery and a DC line; and a control circuit for controlling thecharge-discharge circuit, wherein the battery has a number of batterycells and cylindrical portions of the battery cells are laid on anapproximately horizontal plane.

[0009] According to a second aspect of the present invention, used asthe battery cell is a nickel-metal-hydride (NiMH) rechargeable batterycell having a high energy density.

[0010] With this constitution, a thin DC backup power supply system isrealized having a number of nickel-metal-hydride battery cells of asub-C size about 43 mm in height and about 22.5 mm in diameter.

[0011] In particular, in order to supply a rated output power of 700 Wor larger per one DC backup power supply system, 40 or morenickel-metal-hydride battery cells are used, without using seal typelead battery cells which are the bottleneck of thinning the system. TwoDC backup power supply systems can be accommodated in a spacecorresponding to one unit (1U) size of a 19-inch rack. In this case, itis preferable to connect at least two sets of nickel-metal-hydridebattery cells in parallel.

[0012] If three DC backup power supply systems are accommodated in aspace corresponding to 1U size of a 19-inch rack, the rated output powerof 400 W or larger per each system can be obtained by using 20 or moresub-C size nickel-metal-hydride battery cells.

[0013] According to another aspect of the invention, the voltage at theDC line, i.e., at the connection point to the charge-discharge circuit,is set to 51 to 55 V if the full-charged voltage of the battery is 48 Vor higher.

[0014] A rated output power is ensured for 6 minutes or longer to backup a load during power failure by using nickel-metal-hydride batterycells, under the conditions that a temperature of the battery is 10° C.or higher, an internal impedance of the battery is twice or lower thanan initial value and the battery is in a full-charged state.

[0015] Other objects, features and advantages of the invention willbecome apparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a perspective view of a rack in which mounted are DCbackup power supply systems according to a first embodiment of theinvention.

[0017]FIG. 2 is a perspective view of the DC backup power supply systemsof the first embodiment.

[0018]FIG. 3 is a plan view of the DC backup power supply system of thefirst embodiment.

[0019]FIG. 4 is a perspective view showing the structure of a battery(pack) of the DC backup power supply system of the first embodiment.

[0020]FIG. 5 is a perspective view showing the connection between the DCbackup power supply systems of the first embodiment and an informationprocessing apparatus, as viewed from the back of a rack.

[0021]FIG. 6 is a block diagram showing the connection between the DCbackup power supply systems of the first embodiment and an informationprocessing apparatus.

[0022]FIGS. 7A, 7B and 7C are block diagrams illustrating the operationstate of the DC backup power supply system according to an embodiment ofthe invention.

[0023]FIG. 8 is a graph showing the relation between a maximum supplypower of the DC backup power supply system of the first embodiment and abackup time, by using the number of battery cells as a parameter.

[0024]FIG. 9 is a graph showing the relation between the height of UPSof the first embodiment and its output power, as compared toconventional UPSs.

[0025]FIG. 10 is a perspective view of DC backup power supply systemsaccording to a second embodiment of the invention.

[0026]FIG. 11 is a plan view of the DC backup power supply system of thesecond embodiment.

[0027]FIG. 12 is a perspective view showing the structure of a battery(pack) of the DC backup supply system of the second embodiment.

DESCRIPTION OF THE EMBODIMENTS

[0028]FIG. 1 is a perspective view showing the mount state of DC backuppower supply systems (UPSs) according to a first embodiment of theinvention and information processing apparatuses respectively mounted ina 19-inch rack. In FIG. 1, reference numeral 41 represents the 19-inchrack in which information processing apparatuses 26 are accommodated ina space corresponding to the height of 6 shelves×two units=12 units. TwoDC backup power supply systems of the first embodiment are mounted inthe lower most space corresponding to 1 unit. As seen from FIG. 1, thetwo DC backup power supply systems are accommodated in the small space.On the front side of the rack, reference numeral 3 represents a rackmount fitting, reference numeral 4 represents an LED, reference numeral12 represents an air vent, and reference numeral 19 represents a drawerhandle.

[0029]FIG. 2 is a perspective view of the DC backup power supply systemof the first embodiment. As shown, the DC backup power supply system 1has such a size that two DC backup power supply systems can be mountedside by side in a chassis 2 and each system can be pulled outindependently. The width L of the backup power supply system 1 is about225 mm, the height H2 is 44 mm or lower and the depth D is 600 mm orshallower. The height H2 of the chassis is one unit (1U), i.e., about44.45 mm which allows the chassis to be mounted in a 19-inch rack. Therack mount fitting 3 is mounted on the right and left sides of thechassis 2. LEDs 4, air openings 12 and drawer handle 19 are provided onthe front side of the backup power supply system 1 as described earlier.

[0030]FIG. 3 is a top view of the DC backup power supply system 1 withits lid being removed. In FIG. 3, similar constituent elements to thoseshown in FIGS. 1 and 2 are represented by using identical referencenumerals. In FIG. 3, two battery packs 5 are juxtaposed along the depthdirection. Mounted on the front side of these battery packs 5 arecircuit boards for a charge-discharge circuit 6 having a coil 8,electrolytic capacitors 9 and a heat sink 11 and for a control circuit 7having a microcomputer 10. Installed on the backside are a cooling fan13, connectors 20 and 21 and a switch 22.

[0031]FIG. 4 is a perspective view showing the structure of batterycells in the battery pack 5 used with the DC backup power supply system.In FIG. 4, seven battery cells are serially connected by conductivemembers 18 and disposed in a horizontal direction as viewed in FIG. 4,and four sets of such serial connections are disposed in a verticaldirection. An insulated sheet 14 is disposed between adjacent batterycell sets. In this manner, twenty eight battery cells in total are allserially connected between a positive electrode 16 and a negativeelectrode 17. Each battery cell 15 is a sub-C size nickel-metal-hydridebattery (NiMH battery) cell which is about 43 mm in height and about22.5 mm in diameter. The size of the battery pack is about 160 mm in thehorizontal direction, about 180 mm in the vertical direction and about25 mm thick. The battery pack 5 is disposed in the backup power supplysystem, with its thickness direction being directed to the heightdirection of the backup power supply system, i.e., the cylindricalportion of the battery cell 15 is laid on an approximately flat plane.In this manner, the DC backup power supply system having a 1U size inheight can be realized.

[0032]FIG. 5 is a perspective view showing the connection between DCbackup power supply systems of the first embodiment and an informationprocessing apparatus, as viewed from the back of the rack. In FIG. 5,similar constituent elements to those shown in FIGS. 1 to 3 arerepresented by using identical reference numerals. On the back side ofan information processing apparatus 26, two apparatus power sources 25are installed. Installed on the back side of the apparatus power source25 are an AC plug 27 and a power switch 28 as well as a connector 29, ahandle 30 and a cooling fan 31. In addition to these components, on theback side of the information processing apparatus 26, two connectors 33are installed. Two backup power supply systems are mounted in thechassis 2. The chassis 2 and information processing apparatus 26 aremounted in the same 19-inch rack. The apparatus power sources 25 and thebackup power supply systems 1 are interconnected by the connectors 29via DC power/signal cables 24 and connectors 21. The informationprocessing apparatus 26 and backup power supply systems 1 areinterconnected by the connectors 33 via signal cables 23 and connectors20.

[0033] Heat is generated in the backup power supply systems 1 mainlyduring discharge. In such a case, the control circuits 7 make thecooling fans 13 rotate to blow air through a path from the front airvents 12 to the charge-discharge circuits 6, battery packs and to thecooling fans 13 to cool the backup power supply system 1.

[0034]FIG. 6 is a block diagram showing the electric connection betweenDC backup power supply systems 1 of the first embodiment shown in FIGS.1 to 5 and an information processing apparatus 26. In FIG. 6, similarconstituent elements to those shown in FIGS. 1 to 5 are represented byusing identical reference numerals. The internal battery pack 5 of thebackup power supply system 1 is connected to the charge-dischargecircuit 6, and supplies/outputs a power/signal to the control circuit 7for the charge-discharge circuit 6. An output terminal of thecharge-discharge circuit 6 is connected to the connector 21. The controlcircuit 7 is connected to the connectors 20 and 21. As described withreference to FIG. 5, two apparatus power sources 25 are built in theinformation processing apparatus 26. Each of the apparatus power sources25 has an AC/DC converter 34 and a DC/DC converter 35. An AC powersource 32 supplies a DC power to a load 39 via the AC plug 27, AC/DCconverter 34 and DC/DC converter 35. The two apparatus power sources 25have quite the same structure and their outputs are connected inparallel to the load 39. The load 39 includes a control circuit 36, adisk drive 37 and a memory 38. The connection point between the AC/DCconverter 34 and DC/DC converter 35 in the apparatus power source 25 isconnected to the connector 29. A power-good signal from the AC/DCconverter is supplied to the connector 29. The connector 29 is connectedto the connector 21 of the backup power supply system 1 via the DCpower/signal cable 24. The control circuit 36 in the load 39 isconnected to the connector 33 and to the connector 20 of the backuppower supply system 1 via the signal cable 23.

[0035] The control circuit 7 always monitors the state of the batterypack to control a charge current of the charge-discharge circuit. Whenthe battery pack 5 enters a full-charged state, the control circuit 7controls to stop the operation of the charge-discharge circuit 6. Thetwo backup power supply systems 1 have the same structure andindependently monitor the states of the battery packs 5 to performcharge control. By stopping the operation of the charge-dischargecircuit 6 in the full-charged state, a so-called trickle charge can beprevented and the life time of each nickel-metal-hydride battery cellcan be prolonged.

[0036] Consider now the charge of nickel-metal-hydride battery cells 15of 28 cells×two parallel sets=56 cells. A cell having a nominal voltageof 1.2 V rises to 1.6 V in the full-charged state. A voltage across thebattery back 5 having 28 cells is therefore 44.8 V. In this case, if thesetting voltage of the intermediate DC line 29, i.e., the settingvoltage at the output terminal 21 of the backup power supply system 1,is set around 48 V, it becomes impossible to retain the terminal voltageof 44.8 V of the battery pack 5, because of a variation in the DC outputvoltage to be caused by the control error of ±10% of the AC/DC converter34 and by a circuit voltage drop. In the embodiment of the invention,therefore, the center value (design value) of the voltage of theintermediate DC line 29 is set to 54 V with some margin to therebyreliably maintain the charge voltage even under such variation andvoltage drop. There is generally no practical problem if the centervalue is set to 51 to 55 V.

[0037]FIGS. 7A, 7B and 7C are simplified block diagrams illustrating theoperation state of the DC backup power supply system of the embodiment.In FIG. 7, similar constituent elements to those shown in FIGS. 1 to 6are represented by using identical reference numerals. As alreadydescribed with FIG. 6, although two series of commercial AC powersources 32 are used, the same AC power source may be used. One of thetwo AC power sources may be an electric generator or a large capacityuninterrupted power source. While both the commercial power sources 32are in a normal state, the backup power supply system 1 are in a chargestate or a standby state.

[0038]FIG. 7A is a block diagram illustrating the operation during acharge state. There are two power paths. The first power path is a pathfor supplying a power from the commercial AC power source 32 to the load39 via the AC/DC converter 34 and DC/DC converter 35. The second powerpath is a path for charging the battery pack 5 from the output point ofthe AC/DC converter 34 via the charge-discharge circuit 6 of the backuppower supply system 1. It is preferable that the voltage at the outputpoint of the AC/DC converter 34 is about DC 48 V (or 51 to 55 V asdescribed earlier). Although a lower voltage of 12 V or 24 V, which islower as compared to the power capacity of the load 39, may be used, thecurrent increases correspondingly so that the DC power/signal cable 24is required to be thick.

[0039] The voltage at the output point may be set to DC 380 V which isan output of a phase factor circuit (PFC) built in a general AC/DCconverter. In this case, however, it is to be noted that as compared tothe 48 V (or 51 to 55 V as described earlier) series, the insulation ofthe backup power supply system and DC power/signal cable 24 becomesdifficult.

[0040]FIG. 7B is a block diagram showing the power paths under a peakcutfunction according to the embodiment. This peakcut function provides acontrol means for supplying a power also from the backup power supplysystem 1 in addition to a power supplied from the AC/DC converter 34when a load current over a predetermined value flows through the DC/DCconverter 35 while the commercial AC power source 32 is normal. Thiscontrol means is constituted of the charge-discharge circuit 6 andcontrol circuit 7. It is therefore possible to suppress the outputcurrent of the AC/DC converter 34 equal to or lower than thepredetermined value. A user can have the merits that the rated capacityof the AC/DC converter can be suppressed lower, current input from thecommercial AC power source 32 can be suppressed smaller, powerequalization is realized and contract electric power can be reduced.

[0041]FIG. 7C shows the power paths during power failure. In thisembodiment, the discharge operation starts when a change in thepower-good signal is received from the AC/DC converter 34. It istherefore possible to back up the power supply to the load in responseto not only power failure but also AC/DC converter failure, both in thesame manner. The reliability of the system can be improved.

[0042] The operation will be described which is made when the commercialAC power source 32 fails. As the commercial AC power source 32 fails, anoutput voltage of the AC/DC converter 34 lowers. At this time, thepower-good signal supplied from the AC/DC converter changes to anabnormal state signal. This change is similarly applied to failure ofthe AC/DC converter 34. This change in the power-good signal iselectrically sent to the control circuit 7 of the backup power supplysystem 1 via the connector 29, DC power/signal cable 24 and connector21. Upon reception of this change in the power-good signal, the backuppower supply system 1 starts the discharge operation of thecharge-discharge circuit 6. The DC power of the battery pack 5 isconverted into the predetermined 48 V (or 51 to 55 V as describedearlier) by a boost converter of chopper in the charge-discharge circuit6, the converted voltage being applied to the output point of the AC/DCconverter 34. The boost converter of chopper is made of a well-knowncircuit constituted of the coil 8, a semiconductor device such as apower MOSFET mounted on the heat sink 11 and the electrolytic capacitors9.

[0043] As described with reference to FIG. 4, the battery packs 5 of theembodiment are made of two parallel connections of a serial connectionof twenty eight nickel-metal-hydride battery cells. Although the voltageacross the battery packs 5 changes from time to time, the rated voltageis 33.6 V (1.2 V/cell). Although there are some power conversioncircuits for generating a stable 48 V (or 51 to 55 V as describedearlier) from the battery pack, the above-described boost converter ofchopper among others is most simple.

[0044]FIG. 8 is a graph showing the relation between maximum supplypowers of the DC backup power supply system and the battery and a backuptime, by using the number of nickel-metal-hydride battery cells as aparameter. This graph was obtained by using nickel metal hydride (NiMH)battery cells of the sub-C size under the severe conditions of acharge-discharge circuit efficiency of 90%, a cell deterioration with aninternal impedance twice the initial value, and a low temperature of 10°C. As seen from this graph, in order to obtain a backup power supplysystem capable of backing up for 6 minutes at an output power of 700 W,battery cells between 40 cells and 50 cells are required. The graphshown in FIG. 8 shows an output under the worst conditions describedabove. In the initial state of battery cells, 40 cells are sufficientfor the backup for 6 minutes at an output power of 700 W. For theapplication that requires the reliable backup for 6 minutes at an outputpower of 700 W under the worst conditions, it is desired to use 45 ormore battery cells.

[0045] Similarly, as seen from FIG. 8, 20 to 30 cells are required forthe backup for 6 minutes at an output power of 400 W or for 5 minutesfor an output power of 500 W (indicated by broken line). From the samereason described above, it is necessary to use 20 cells or more, or morepreferably about 28 cells.

[0046]FIG. 8 also shows the backup capacity of the DC backup powersupply system using 28 cells×two parallel connections=56 cells of thenickel-metal-hydride battery cells 15 of the first embodiment. The samesevere conditions were applied, i.e., the conditions of acharge-discharge circuit 6 efficiency of 90%, a cell deterioration withan internal impedance twice the initial value, and a low temperature of10° C. As seen from the graph, the maximum backup output is about 100W×3 min or shorter, about 920 W×5 min (indicated by a broken line),about 880 W×about 6 min, or about 880 W×about 6 min which is about 790W×6 min when the power supply system efficiency is taken intoconsideration. These output capacities sufficiently surpass that ageneral 1 kVA AC type UPS can backup 670 to 700 W×6 min in the initialstate, and are compatible with the specification that a 1.2 kVA ACoutput type UPS can back up 840 W×6 min in the initial state. The backuppower supply system of the first embodiment has the backup capacitygenerally equivalent to that of the 1.2 kVA AC output type UPS.

[0047] In the backup power supply system 1 of the invention, theefficiency of the backup power supply system 1 can be improved more thanan AC output type UPS by the amount corresponding to the efficiency ofthe AC/DC converter because the AC/DC converter does not feed powerduring the backup. In other words, the backup power supply system havingthe same capacity as that of an AC output type UPS can prolong thebackup time more than AC output type UPS by the amount corresponding tothe efficiency of the AC/DC converter. For example, assuming that theefficiency of an AC/DC converter is 90% and the backup time of an ACoutput type UPS is 6 min, then the backup time of a backup power supplysystem having the same capacity becomes 6 min/0.9=6.6 min.

[0048] In this embodiment, the switching from a power failure to anoutput of 48 V (or 51 to 55 V as described earlier) can be performed inseveral hundreds μs. Therefore, an input to the DC/DC converter 35 doesnot change greatly so that the DC/DC converter 35 operates independentlyfrom the power failure. The load 39 can continue its operation stably.

[0049] If power failure is recovered in relatively short time and thecommercial AC power source 32 recovers, the power-good signal of theAC/DC converter 34 changes from the abnormal state to the normal state.This signal change is detected by the control circuit 7 to stop thedischarge.

[0050] The remaining capacity (SOC) of the battery pack is alwaysmonitored by the control circuit 7. SOC can be estimated mainly throughcumulative addition of charge current or discharge current to and fromthe battery pack 5. If SOC of the battery pack 5 lowers because of longpower failure, the control circuit 7 outputs a shut-down signal to thecontrol circuit 36 which in turn enters a shutdown operation. Thisoperation is, for example, an operation of saving the contents of thememory 38 into the disk 37.

[0051] After the shutdown operation, the control circuit 36 sends an UPSshutdown signal to the control circuit 7 which in turn stops thedischarge of the battery pack.

[0052] Next, the description will be made on the operation to beexecuted when the backup power supply system 1 fails. When the backuppower supply system 1 fails, the control circuit 7 stops the operationof the charge-discharge circuit 6 and indicates an alarm on LEDs 4.Although not shown, this alarm may be effected by using a buzzer builtin the backup power supply system 1. A failure of the backup powersupply system 1 may be notified to the control circuit 36 to notify itto the user from the system side. After the user recognizes the failureof the backup power supply system 1 in this manner, the backup powersupply system 1 is replaced with a new one. In the replacement processof the backup power supply system 1, the switch 22 on the back side isturned off and the cables are pulled out to draw out the system with thehandle 19. Next, the normal backup power supply system 1 is inserted asshown in FIGS. 1 and 2, and the cables and the like are wired to theconnectors to thereafter turn on the switch as shown in FIG. 4. In thiscase, the operation of the information processing apparatus as the loadis not required to be stopped. In this embodiment, as shown in FIG. 6,since the backup power supply systems 1 are connected in parallel to thepath from the commercial AC power source 32 to the load 36, the backuppower supply system 1 can be pulled out or inserted while the load ismaintained to operate.

[0053]FIG. 9 is a graph showing the size of the DC backup power supplysystem of the first embodiment as compared with the sizes ofcommercially available several rack mount type AC output type UPSs. Thisgraph shows the relation between the output power W of the backup powersupply system and a UPS height. As plotted by white circles in FIG. 9,since the height of a rack mount AC output type UPS is generally about44 mm (1U), about 88 mm (2U), and so on because the height pitch of19-inch racks is about 44.45 mm (1U). An AC output type UPS having thelowest height of 1U exists only for a relatively small output power of400 W or smaller because there is a bottleneck of reducing the volume ofa transformer, an inverter and a seal type lead battery. Each of rackmount AC output type UPSs of 1 to 1.4 kVA (approximately correspondingto 700 to 1000 W), which are manufactured in large units, has a heightof 2U to 3U. Information processing apparatuses are becoming thinneryear after year in order to increase the packaging density. 1U sizeservers have been presented recently and there is a high need ofincreasing the packaging density of the whole of a rack.

[0054] As plotted by a black circle (a) in FIG. 9, according to thefirst embodiment, two rack mount DC output type UPSs of 1 to 1.4 kVA(approximately corresponding to 700 to 1000 W), which are manufacturedin large units, can be accommodated in a 1U space.

[0055] A DC backup power supply system according to the secondembodiment of the invention will be described with reference to FIGS. 10to 12.

[0056]FIG. 10 is a perspective view of DC backup power supply systemsaccording to a second embodiment of the invention. This structure isbasically the same as that shown in FIG. 2, and so only different pointswill be described. In the example shown in FIG. 10, three backup powersupply systems can be mounted on a chassis 2. The height H2 of thechassis 2 is 1U which is the same as FIG. 1. The height Hi of the backuppower supply system 1 is 44 mm or lower which is the same as FIG. 1. Thewidth L2 of the backup power supply system 1 is 150 mm or shallower.

[0057]FIG. 11 is a plan view showing the internal structure of the DCbackup power supply system 1 of the second embodiment. Although thesizes of the charge-discharge circuit 6 and control circuit 7 are thesame as those shown in FIG. 3, L2 is narrow , 150 mm or narrower so thatthe control circuit 7, charge-discharge circuit 6 and battery pack 5 aredisposed in this order from the front side.

[0058]FIG. 12 is a perspective view showing the internal structure ofthe battery pack 5. The battery pack 5 is constituted of thirty sub-Csize nickel-metal-hydride battery cells serially connected, without aparallel connection of battery packs 5. Five battery cells are arrangedin one row and six battery rows are stacked in the vertical direction,with an insulated sheet 14 being sandwiched between adjacent batterycell rows. A positive electrode 16 and a negative electrode 17 areprovided on opposite ends of the cell array structure. The size of thisbattery pack is about 270 mm in the vertical direction, about 115 mmwide and about 25 mm thick. This battery pack can be accommodated in aspace having the size shown in FIG. 11.

[0059] The operation of the backup power supply system 1 of the secondembodiment is the same as that of the first embodiment. The capacity perone backup power supply system is different between the first and secondembodiments. This point will be described with reference to FIG. 8. Thebattery pack 5 contains thirty sub-C size nickel-metal-hydride batterycells. The following maximum backup outputs can be obtained under theconditions of a charge-discharge circuit 6 efficiency of 90%, a celldeterioration with an internal impedance twice the initial value, and alow temperature of 10° C. As seen from the graph, the maximum backupoutput is about 550 W×3 min or shorter, or about 480 W×about 6 min whichis about 430 W×6 min when the power supply system efficiency is takeninto consideration. These output capacities surpass the output power ofa 500 VA (350 W) AC output type UPS and are compatible with the initialperformance of a 700 VA (490 W) AC output type UPS. The backup powersupply system 1 of the second embodiment has the ability to back up thecapacity guaranteed as the initial value of the 700 VA AC output typeUPS.

[0060] As plotted by a black circle (b) in FIG. 9, three DC backup powersupply systems 1 of the second embodiment having the backup ability of430 W×6 min can be accommodated in the space corresponding to one unit(1U) size of a 19-inch rack.

[0061] Each of thirty nickel-metal-hydride battery cells 15 has 1.6 Vnear the full-charged state. The voltage across the terminals of thebattery pack 5 is therefore 30×1.6 V=48 V. In this case, the settingvoltage at the intermediate DC line 29, i.e., the setting voltage at theoutput terminal 21 of the backup power supply system 1, is required tobe set to a voltage at least higher than by an amount corresponding tothe circuit voltage drop. Also in the second embodiment, the voltage atthe intermediate DC line 29 is set to 54 V so that the charge voltagecan be retained with some margin even if there are the regulationvariation of the AC/DC converter 34 and the circuit voltage drop.

[0062] Also in the backup power supply system of this embodiment, eachfunction of charge, discharge and peakcut can operate in the similarmanner as that of the first embodiment.

[0063] According to the above-described embodiments, by replacing a rackmount AC output type UPS by a backup power supply system of theembodiments, the backup function of the same capacity can be realized bya thinner size, 1U size. It becomes possible to increase the number ofload systems such as systems, information processing apparatuses andservers. The capacity of such a load system can be increased so that thepackaging density of a rack can be increased.

[0064] Also in the above embodiments, when rechargeable battery cellsand other components in the backup power supply system are to bemaintained and replaced, each backup power supply system can behot-swapped through connection and disconnection of the connectors andcables on the back side. Replacement works can be performed while theload is maintained to operate and is not turned off.

[0065] With the peakcut function of the backup power supply system, itis possible to realize input power equalization and reduce a contractelectric power, a rated capacity of the AC/DC converter and a cost.

[0066] By using not a seal type lead battery but a nickel-metal-hydridebattery, the environment load when lead is otherwise dumped can bemitigated and a safe system can be provided.

[0067] According to the invention, a DC backup power supply system canbe realized in a thin size and can be mounted easily on a rack havingsystems, information processing apparatuses, servers and the like.

[0068] If a seal type lead battery is not used but anickel-metal-hydride battery is used, the environment load when lead isotherwise dumped can be mitigated and a safe system can be provided.

[0069] It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

What is claimed is:
 1. A DC backup power supply system comprising: abattery; a charge-discharge circuit for charging and discharging a powerbetween said battery and a DC line; and a control circuit forcontrolling said charge-discharge circuit, wherein said battery has anumber of battery cells and cylindrical portions of the battery cellsare laid on an approximately horizontal plane.
 2. A DC backup powersupply system comprising: a battery; a charge-discharge circuit forcharging and discharging a power between said battery and a DC line; anda control circuit for controlling said charge-discharge circuit, whereinsaid battery has a number of battery cells, cylindrical portions of thebattery cells are laid on an approximately horizontal plane, and aheight of said battery is 45 mm or lower.
 3. A DC backup power supplysystem comprising: a battery; a charge-discharge circuit for chargingand discharging a power between said battery and a DC line; and acontrol circuit for controlling said charge-discharge circuit, whereinsaid battery has a number of battery cells, cylindrical portions of thebattery cells are laid on an approximately horizontal plane, and saidbattery is accommodated in a space having a height corresponding to oneunit (1U) size of a 19-inch rack.
 4. A DC backup power supply systemaccording to claim 3, wherein two DC backup power supply systems eachhaving a rated output power of 700 W or larger are accommodated side byside in the space having the height corresponding to the one unit (1U)size.
 5. A DC backup power supply system according to claim 1, whereinsaid battery has 40 or more sub-C size nickel-metal-hydride batterycells.
 6. A DC backup power supply system according to claim 5, whereinthe nickel-metal-hydride battery cells include two or more parallelconnections of nickel-metal-hydride battery cells.
 7. A DC backup powersupply system according to claim 3, wherein three DC backup power supplysystems each having a rated output power of 400 W or larger areaccommodated side by side in the space having the height correspondingto the one unit (1U) size.
 8. A DC backup power supply system accordingto claim 7, wherein said battery has 20 or more sub-C sizenickel-metal-hydride battery cells.
 9. A DC backup power supply systemaccording to claim 1, wherein if a full-charged voltage of said batteryis 48 V or lower, a voltage at the DC line is set to 51 to 55 V.
 10. ADC backup power supply system according to claim 4, wherein the ratedoutput power is supplied for 6 minutes or longer under the conditionsthat a temperature of said battery is 10° C. or higher, an internalimpedance of said battery is twice or lower than an initial value andsaid battery is in a full-charged state.
 11. A DC backup power supplysystem comprising: a battery; a charge-discharge circuit for chargingand discharging a power between said battery and a DC line; and acontrol circuit for controlling said charge-discharge circuit, whereinsaid battery has a number of battery cells, cylindrical portions of thebattery cells are laid on an approximately horizontal plane, a pluralityof DC backup power supply systems electrically connected in parallel areaccommodated and juxtaposed in one rack, and the DC backup power supplysystems have electric hot swap connections using connectors and cables.12. A DC backup power supply system according to claim 1, furthercomprising control means, when a power supplied from the DC line andconsumed by a load exceeds a predetermined value, said control meansactivates the DC backup power supply system to supply a power to theload.